Bring malt vinegar and fenugreek seeds to a boil in a non-reactive saucepan. Remove from heat and set aside for 6 hours or overnight.
Dry-roast cumin seeds over gentle heat until fragrant. Cool, then grind to a fine powder. Blend all ingredients to a smooth, fine paste in an electric blender, in small batches if necessary.
Cook paste in a wide, heavy-based pan over gentle heat f